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Quantum Mechanical Atom

Contrary to the way they're often depicted, electrons don't orbit wildly around the nucleus of an atom. Their movements are orderly and prescribed by the energy level they inhabit. This illustrated essay, written for Teachers' Domain, describes the tidy configuration of electrons around a nucleus and explains how this arrangement is determined for each element, based on the number of electrons it has.

At first glance, the periodic table -- that chart that appears on the walls of science classrooms everywhere -- appears to be an oddly shaped collection of chemical information about the elements. A closer look, however, reveals the source of the table's name: The elements are arranged "periodically;" that is, according to properties that repeat in regular, predictable patterns. This periodic arrangement of the elements makes the table very useful, in that if you know the location of an element in the table, you can predict its properties.

The more than 100 elements that make up the periodic table are organized in a series of 18 columns and 7 rows. Each column is called a group, or family. Each row is called a period. Elements in the same group have similar physical characteristics. For example, all of the elements in group 1 (at the far left) react easily with other elements. Unlike the elements in a group, however, the elements in a period do not share properties. Rather, the properties of the elements change as you move from left to right across the row. But to understand why the table is organized as it is, it's helpful to understand the structure of atoms.

An atom is the smallest particle of an element. An atom of any given element is made up of a certain number of protons, an equal number of electrons, and approximately the same number of neutrons. (The exception is hydrogen, which can have zero neutrons.) Protons and neutrons form the nucleus of an atom, and electrons swarm around the nucleus. This swarming isn't completely haphazard, though. Electrons inhabit various energy levels, or shells. The electron configuration shown in the periodic table indicates how many electrons are found in each shell, from innermost to outermost. For example, the electron configuration for calcium is 2, 8, 8, 2.

Electron configuration depends upon the energy state and magnetic spin of each electron, and these qualities place electrons into particular subshells within each shell. The first shell, for example, includes only one subshell at the lowest-level energy state and can hold no more than two electrons. The second shell, with two subshells that contain four levels of energy states, can hold no more than eight electrons. The subshells containing the lowest energy states fill first, and if a subshell is full, additional electrons are found in the next higher subshell -- which is generally in the adjacent outer shell.

Beginning in the fourth shell, there are subshells that have lower energy states than those in the adjacent inner shell. Since the electrons fill the levels in order of energy, electrons can start filling subshells in an outer shell before an inner shell is completely full. This explains why, for example, the electron configuration for calcium can be 2,8,8,2 when the third shell can hold up to 18 electrons.

Elements are arranged in the periodic table according to atomic number, from left to right, top to bottom. The atomic number of an element is equal to the number of protons found in an atom of that element. For example, an atom of carbon has six protons in its nucleus; its atomic number is 6. The elements are also arranged according to atomic mass. The mass of a single proton is equal to 1, while the mass of a neutron is very close to 1. An atom's atomic mass, then, is close in number to the sum of its protons and neutrons. An atom of carbon, with six protons and, on average, six neutrons in its nucleus, has an atomic mass of 12.0107. With the lighter elements, the atomic mass is about double the element's atomic number. As you move up to the heavier elements, the number of neutrons relative to protons increases, causing the mass to be increasingly more than double the atomic number.

When Dmitri Mendeleyev first devised the modern periodic table in 1869, he organized it such that elements with similar characteristics fell into the same columns. Doing so naturally created rows within the table. What scientists later found out was that these rows represented something very significant. They discovered that the elements in each successive row contained an additional electron shell. For example, the atoms of hydrogen and helium in the first row each had one electron shell; atoms of elements listed in the second row had two electron shells, and so on to elements in the final row, whose atoms each have seven shells.

From this, scientists learned what caused elements to have different characteristics. Each element's physical characteristics are determined, in large part, by the number of electrons in the outermost shell of its atoms. As with the number of protons, the number of electrons increases by one as you move across the table from left to right, top to bottom. Atoms of elements in the left-hand column have one electron in their outer shell, while atoms of elements in the right-hand column have eight electrons in their outer shell. How does this determine an element's characteristics? Single electrons in an outer shell can easily be taken away from the atom with the application of very little energy. This makes atoms of elements in the left-hand column very reactive (and good conductors of heat and electricity). It is very difficult, on the other hand, to add or remove electrons from an atom that has eight electrons in its outer shell. The atoms of these elements, found in the column to the far right, are non-reactive.

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